Thirty to forty percent of the human population under age 40 develop an ocular refractive error requiring correction by glasses, contact lenses, or surgical means. Refractive errors result when the primary optical elements of the eye, the cornea and the lens, fail to image incoming light directly on the retina. If the image is focused in front of the retina, myopia (nearsightedness) exists. If the eye image is focused behind the retina, hyperopia (farsightedness) exists. The focusing power of the eye or any of the eye's individual components is measured in units called diopters.
Approximately 20% of the patients under 40 having vision defects cannot wear contact lenses because the contact lenses do not fit (become dislodged and/or are very uncomfortable), or they fail to provide the requisite optical correction, or both. In addition, many patients who currently wear contact lenses are not satisfied with the length of time they can wear their lenses and/or with the visual acuity their contact lenses provide.
Over age 40, the percentage of the population requiring vision correction dramatically increases because the crystalline lens of the eye becomes relatively inelastic. With increasing age the quality of the tear film decreases as well as the rate of lid blink that causes some previously successful contact lens wearers such discomfort that they may abandon their lenses.
Contact lenses are customarily described as “corneal” wherein the lens is of such diameter that it rests purely on the corneal surface and “soft” wherein the lens is extended beyond the cornea to rest on the adjacent sclera. Standard contact lenses are rotationally symmetrical and the optical portion of the lens is composed of simple Guass optics, i.e., either a simple sphere or a toric. In the case of “soft” contact lenses vault the cornea as the lens rests on that portion of the eye peripheral to the cornea, i.e . . . the sclera, and vault from the sclera. The human cornea and sclera, however, are “asymmetrically aspheric” surfaces.
“Aspheric” means that the radius of curvature along a corneal “meridian” (which is an imaginary line on the corneal surface passing through the geometric center of the cornea, analogous to a geographic meridian) is not a constant. Indeed, the corneal curvature tends to flatten progressively from the geometric center to the periphery. “Asymmetric” means that the profile of the corneal curvature along a half-meridian is not the same as (i.e., it is not a mirror image of) the other half of the same meridian. Additionally, “asymmetric” means that the profile of the corneal curvature about a central point (i.e., an origin) is not the same as the corneal profile on the opposite side of the central point. The degree to which the cornea is aspheric and/or asymmetrical varies from patient to patient and within the same person.
Current state of the art contact lenses do not match the corneal curvature and geometry, and therefore do not fit properly. The more irregular the patient's cornea the worse the fit, such that about 20% of the patients under age 40 are unable to wear standard contact lenses.
In addition, standard contact lenses are rotationally symmetrical. Sometimes the fitter will generate toric, bitoric and like surfaces in his effort to fit lenses on the cornea. These more complicated lens designs remain inherently rotationally symmetric, i.e., the surfaces are generated about a central point of revolution.
Because the human cornea has an asymmetrically aspheric surface, purely Gauss based lenses poorly match the corneal curvature and geometry. When the lens is designed as a toric lens, the resultant lens surfaces remain rotationally symmetrical (i.e., these lenses are not asymmetrical and aspheric). In some eyes the discrepancy between the lens and underlying cornea's asymmetry is so great that toric lenses fail to center on the cornea and/or give satisfactory vision.
In an effort to alleviate these problems, manufacturers developed lenses with varying curvatures on their posterior surface. For example, U.S. Pat. No. 5,114,628 discloses aspherical contact lenses made using corneal topographic data to control a lathe. (The data, as taught in the '628 patent, provide information on the slope of the corneal surface at different points on the cornea and are based on measurements in two dimensions, interpreted three-dimensionally.) The resultant lens is aspherical (in both the anterior and posterior surface) but inherently symmetrical.
U.S. Pat. No. 2,264,080 to Hunter discloses a system for manufacturing a “contoured” scleral contact lens, i.e., a lens resting outside and intentionally vaulting the cornea. Hunter teaches the creation of a mold of the surface of the eye which is then used as a “template” to mechanically radially guide a grinder over the surface of a lens blank.
Accordingly, there is a need in the art for a better fitting contact lens that will decrease or eliminate the number of patients of all ages who currently cannot wear contact lenses, and provide better comfort for patients who now wear contact lenses.
U.S. Pat. Nos. 5,502,518 and 5,570,142 both to Lieberman and Grierson, which are assigned to the same Assignee as the present invention, are both directed to contact lenses that have posterior surfaces that accurately match at least a portion of the surface of the cornea. The '518 and '142 patents satisfied the need for better fitting contact lens.
U.S. Pat. No. 5,953,098 to Lieberman and Grierson, which is assigned to the present assignee, provides a further refinement of the '518 and '142 patents and provides increased acuity by dividing the surface of the lens into a plurality of segments, each of which has a relatively small surface area so that, particularly in the lens' optical region, the posterior surface of the lens will more closely conform to or match the surface of the underlying cornea resulting in negation of the lens effect of the tear film and, hence, improved acuity.
In U.S. Pat. No. 6,340,229 to Lieberman and Grierson, assigned to the assignee of the present invention, a contact lens is designed which includes a central optical portion and a transition portion radially outward of the central portion and overlying the cornea, which is conformed to the local geometry of the underlying portion of the cornea, including corneal tilt and distortion. In a soft contact lens, a scleral skirt is also provided and the transition portion connects the central optical portion and the scleral skirt.
The disclosures of U.S. Pat. Nos. 5,502,518; 5,570,142; 5,953,098 and 6,340,229 are hereby incorporated by reference in their entirety. In the case of inconsistencies, the present description, including definitions, will control.
Although significant improvement in comfort resulted from the inventions of the patents identified in the preceding paragraph, they necessitated the creation of custom lenses, which requires special equipment and skill to analyze the cornea and design the lense. Such lenses are therefore relatively expensive, and the large majority of lens wearers do not wear them. Instead, they wear contact lenses that are prescribed by an eye care professional on the basis of a conventional eye refraction test and a further measurement which determines the lens required to vault the cornea. Such lenses will hereafter be referred to as “prescription” lenses. Patients continue to have difficulty in wearing prescription contact lenses, particularly for extended periods of time. Patients with certain corneal defects, such as keratoconus, were often even not able to maintain prescription contact lenses in their eyes. That is, the lense centers poorly or in some cases dislodges from the eye.
In an effort to improve lens comfort by reducing the foreign body sensation caused by the lid/cornea interface, many patients now wear soft contact lenses. However, these lenses have not reached their potential for comfort for the same reasons already discussed. In addition, with soft lenses, the mismatch shape between the lens periphery and the eye also results in flexure of the lens, which has a deleterious effect on visual acuity producing frank blurring of vision.
As a result of analysis of such patient difficulties, by utilizing surface modeling methods disclosed in U.S. Pat. No. 6,149,609 and No. 6,416,179, the present inventors have discovered that the peripheral portion of the prescription contact lens accounted for these problems. Prescription lenses are typically formed on a lathe, and the periphery is round and generally lies in a plane. Since the cornea is aspherical and asymmetric at all diameters, a poor fit results particularly at the periphery of the lens. It is a goal of the present invention to improve the fit of prescription contact lenses to permit a greater proportion of the population to wear them comfortably.
In accordance with the present invention, eye measurements taken on a patient being fitted for contact lenses will include an additional measurement representing corneal depth variation on the surface of an imaginary cylinder having the same diameter as the lens. This information can then be used by a lens manufacturer to produce lens in which the perimeter is not circular and does not lie in a plane, but which drops backwardly towards the eye at the appropriate points to produce a saddle-shaped perimeter.